This invention relates to a thermal transfer recording sheet which can print various still pictures such as those picked up by a video camera and viewed on a TV screen, those used in personal computers, etc., as hard copies. More particularly, this invention relates to a thermal transfer recording sheet which can give a color copy by sublimation transfer of a sublimable dye to an image-receiving sheet.
As recording methods for giving color images, there have been used an electro-photographic method, an ink-jet method, a thermal transfer recording method, etc. The thermal transfer recording method is advantageous in that no noise is produced and maintenance of the apparatus is easy. The thermal transfer recording method is a recording method comprising using a solidified-color ink sheet and an image-receiving sheet, and forming images on the image-receiving sheet by hot-melt transfer or sublimation transfer of the ink with thermal energy controlled by electric signals using laser, a thermal head, or the like. In the thermal transfer method, there are a hot-melt transfer method and a sublimation transfer method using sublimable dyes. According to the hot-melt transfer method, an ink paper obtained by bonding a pigment or dye with thermally molten wax is used, and the pigment or dye together with wax melted by thermal energy of a thermal head is transferred to an image-receiving sheet. Therefore, there are defects in that it is difficult to obtain a half-tone necessary as image quality, and a good hue cannot be obtained due to the transferred wax.
On the other hand, the sublimation transfer method, using sublimable dyes applies a conventional sublimation transfer textile printing technique, uses a transfer sheet obtained by, in general, binding a relatively sublimable disperse dye as the sublimable dye with a binder, and obtains a color image by subliming the sublimable dye with heat energy of a thermal head and transferring it to an image-receiving sheet. Since the sublimable dye sublimes corresponding to the heat energy of the thermal head, this method has an advantage in that the half-tone is easily obtained. An important thing in the sublimation transfer method is the ink composition. Further, the most important thing which must be taken care of in the preparation of the ink composition is the selection of a proper binder. It is undesirable that a binder is molten or increases its viscosity remarkably by the heat at the time of transfer, and in such a case, the binder resin is also transferred to an image-receiving sheet to which the ink is transferred. As the binder, the use of nylon type polyamides is disclosed in, e.g., JP-A (Kokai) Nos. 59-14994 and 59-71898. Nylon can give a very tough film but is disadvantageous in that it has a high water absorption rate and is hardly dissolved in a solvent, etc. Further, in order to effectively use the heat energy of the thermal head, a thin polymer film of 6 .mu.m or less in thickness is used as a substrate in place of condenser paper, tissue paper, or a polymer film of 8 .mu.m in thickness. In such a case, the adherence of the film and the ink layer becomes a problem. Nylon is not so good in adherence. That is, when the adherence to the film is not good, the ink layer per se is transferred to the image-receiving sheet by the heat of thermal head, resulting in causing an undesirable, abnormal transfer phenomenon.